In recent years, as one of next-generation flat panel displays in place of liquid-crystal display apparatuses, attention has been given to organic electroluminescence (EL) display apparatuses which use EL of organic materials. An active-matrix display device such as an organic EL display apparatus includes thin-film transistors (TFTs) in respective pixels arranged in rows and columns, which form a TFT array substrate.
Each of the thin-film transistors includes, above the substrate, a gate electrode, a semiconductor layer (a channel layer), a source electrode, and a drain electrode, and is used as a driver transistor or a switching transistor in each of the pixels of the TFT array substrate. Furthermore, a metal of which each electrode of the thin-film transistor is made is used also as a wiring line; the metal of the source electrode is used also as a source line, and the metal of the drain electrode is used also as a drain line, for example. In other words, the source electrode or the drain electrode and the source line or the drain line are formed by patterning of the same metal film.
Recent years have seen a demand for a decrease in line resistance along with an increase in size of a screen of the display apparatus and have developed studies on a copper line which uses low-resistance copper (Cu) as a material of the source line or the drain line. In this case, the source electrode or the drain electrode which is formed in the same layer as the source line or the drain line will be formed of copper.
However, the use of copper as a material of the source electrode or the drain electrode causes a problem of thermal diffusion of copper which occurs due to heat taken in during a thermal process or heat generated during an operation of TFT, resulting in an adverse effect of diffused copper on the semiconductor layer. Thus, in order that copper is prevented from being diffused into the semiconductor layer, there is conventionally a proposed technique of forming, between the source electrode (the drain electrode) and the semiconductor layer, a barrier layer which includes a molybdenum film or a conventional molybdenum nitride film.
Furthermore, PTL 1 discloses a thin-film transistor in which, under a protective film, a cover layer containing silicon is formed as a barrier layer in order that copper which forms a copper line on the protective film is prevented from being diffused into a lower conductive layer.